It seems to me that the exposures are different enough to render the test result at least debatable?
The second image looks brighter...
Pictures have no EXIF data. If you used the same parameters, it could be a difference in light (in the first one you seem to have a shadow on the forehead of the skull), ISO (it is my understanding that ISO settings are not exactly the same on every camera, and the same standard value (es. "ISO800") actually can mean a slightly different value, approximated to the displayed one), transmittance (t-stops) etc.
It's a fascinating thing to compare lenses!